1. Field of the Invention
This invention is directed generally to a coordinate detection device employing a plurality of infra-red light beams, and particularly to such a device suitable for a touch control for a display device having a curved display surface.
2. Description of the Prior Art
A touch panel system employing a plurality of light beams is well-known and described in U.S. Pat. Nos. 3,764,813, 3,775,560, etc. Such a touch panel system generally consists of a coordinate detection device and a display device, and forms a man-machine interface for many kinds of computer systems.
Specifically, as described in the above-mentioned U.S. patents, a plurality of light emitting diodes (LED) are linearly arranged on a printed circuit board located on one side of the display screen of a display device such as a cathode ray tube (CRT) and emit infra-red light beams which are received by a plurality of photo-transistors linearly arranged on a printed circuit board located on the opposite side of the display screen. Further, a plurality of light emitting diodes (LED) linearly arranged on a printed circuit board located on the bottom side of the display screen emit infra-red light beams which are received by a plurality of photo-transistors linearly arranged on a printed circuit board located on the top side of the display screen, thereby forming a grid of the infra-red light beams. Each of the LED and photo-transistor pairs is assigned a different address.
It can be known which of the LEDs emits the light beam and which of the photo-transistors on the opposite side detects the light beam by sequentially specifying the address to change the respective LEDs and the photo-transistors coupled therewith to form the pairs. A touch on the display screen with a finger or a pen causes interruption of a certain infra-red light beam. The X and Y coordinates at the location where the light beam is interrupted are transferred to a host computer to determine the touched position. The touch panel system is constructed such that the light beam is interrupted by a touch on the optical grid plane formed of the infra-red beams so that the sensing plane (optical grid plane) is made even, i.e. flat.
Since the above-mentioned conventional optical touch panel device has an even sensing plane, if it is combined with a CRT having a curved display surface, a defect is encountered in that parallax is caused in peripheral portions of the CRT. For this reason, instructions for input data cannot be displayed on the CRT in a fine manner. Also, there is the possibility of malfunction. Such a parallax is caused by the fact that the infra-red light beams project in straight lines over the curved display screen of the CRT so that the infra-red light beams pass near the display screen at its central portion but far from the display screen in the vicinity of its edges. Therefore, even if the operator touches a point near an edge of the display screen, parallax makes it difficult to interrupt the infra-red light beam corresponding to the desired point to be touched.
As a parallax free touch panel system, there has been proposed a technique described in U.S. Pat. No. 4,198,623. However, the technique shown in the above U.S patent specification makes the control circuit complicated and also provides only insufficient countermeasures against the parallax. Moreover, the increased thickness of the coordinate detection system gives the operator unsuitable feeling.